This invention relates to fuse arrangements for breaking an electric circuit in response to an overcurrent or high temperature or the like and to an electrolytic capacitor containing a fuse.
Conventionally, circuit boards incorporated a fuse arrangement along with other components in order to break the circuit when, for some reason, excess current flows in the circuit, thereby preventing malfunction of the circuit or a resulting disaster such as a fire, or the like.
FIGS. 23 and 24 illustrate one such fuse arrangement of this type which is disclosed in Japanese Patent Publication No. Sho-63-170826. In that arrangement, an electrically insulating substrate 10, made of alumina or the like, is provided at opposite sides with a pair of connection terminals 11. An electrical and thermal insulating layer 12 is applied to one surface of the insulating substrate 10 and a fuse member 13 extending between the terminals 11 is formed on the insulating layer 12. The fuse member 13 is made by applying a coating of conductive paste consisting of a mixture of fusible insulating particles, conductive particles and a binder to the layer 12 which is then fired. The insulating layer 12 has the same characteristics as the fusible insulating particles included in the fuse member 13. Thereafter, a glass protective layer 14 is formed over the fuse member 13.
With this fuse arrangement, when the current flowing between opposite ends 13a and 13b of the fuse member 13 exceeds a predetermined level, the fusible conductive and insulating particles of a fuse element 13c are locally melted by the heat produced as a result, and the conductive particles in the fuse element are thus separated and dispersed from each other, so that conductive paths between the conductive particles in the element are destroyed. At this time, since the insulating layer 12 has the same characteristics as that of the fusible insulating particles of the fuse member 13, the molten conductive and insulating particles of the fuse member 13 are dispersed into the insulating layer 12, so that the conductive paths are eliminated quickly and effectively.
Furthermore, the insulating layer 12 has the following additional function. The electrical insulating substrate 10 is made of a ceramic material such as an alumina material, Al.sub.2 O.sub.3, and the thermal conductivity of such ceramic materials is high enough that the heat resulting from an excessive current in the fuse member 13 is conducted and dispersed through the insulating substrate if the fuse member is formed directly on the insulating substrate. As a result, it is difficult to melt and break the fuse member 13 even if excessive current flows in that member. The electrical and thermal insulating layer 12 prevents heat from the fuse member from being conducted or dispersed into the electrical insulating substrate and into the air. Likewise, the glass protective layer 14 prevents the heat generated in the fuse member from being dispersed into the air.
FIG. 25 is a sectional view illustrating the structure of another conventional fuse arrangement which is disclosed in Japanese Unexamined Utility Model Publication No. Sho-56-92347. In that fuse arrangement, a metal wire 16 is connected to the ends of two terminal portions 15 which extend into the fuse body. The metal wire 16 is coated with a resilient and incombustible or fire-resistant resin 17, and the structure is encapsulated within a molded resin casing 18. When the current flowing through the wire 16 between the lead terminals 15 of the fuse body exceeds a predetermined value, the metal wire 16 melts and disintegrates so that the electrical connection between the terminals 15 is broken. The resilient and incombustible or fire-resistant resin 17 enclosing the metal wire 16 is provided so that, when the metal wire 16 is melted and disintegrates, the disintegrated portion of the metal wire 16 is compressed into a compact configuration by surface tension. In addition, the resin casing 18 prevents heat from the metal wire 16 produced by the excess current from being dispersed.
The conventional fuse arrangement of the type shown in FIG. 25 may also have a surface-mount type of structure as shown in FIG. 26.
In order to make this type of fuse arrangement operate quickly and effectively when the current flowing therein exceeds a predetermined value, it is necessary to take into account the following two points: (1) The temperature of the fuse element produced by a predetermined current value should always be the same. In other words, the dissipation of heat generated in the fuse element when the current exceeds a predetermined value should be prevented as much as possible in order to keep the breakdown characteristics of the fuse element constant. (2) The fuse element must disintegrate sufficiently at the breakdown temperature that the disintegrated portion of the fuse element cannot return to a connected condition upon solidification.
For this reason, the above-mentioned conventional fuse arrangement includes a thermal and electrical insulating layer between the electrical insulating substrate and the fuse element, a glass protective layer on the fuse element, an encapsulating resin covering the whole fuse, and the like, in order to prevent heat produced by excessive current in the fuse element from being conducted away and dissipated. In addition, in order to make the fuse element disintegrate quickly and effectively, the thermal and electrical insulating layer between the electrical-insulating substrate and the fuse element has the same characteristics as that of fusible insulating particles included in the fuse element and the fuse element is enclosed in a resilient and incombustible or fire-resistant resin, and the like. In such conventional fuses, however, the arrangement for preventing the heat generated by excess current in the fuse element from being conducted away and dispersed, and the arrangement for making the fuse element disintegrate quickly and effectively are provided separately. Accordingly, the number of manufacturing steps is large and the method of manufacturing is complicated so that the manufacturing cost is increased.
Furthermore, as shown in FIG. 27, a conventional solid electrolytic capacitor provided with a safety fuse, disclosed in Japanese Unexamined Patent Publication No. Hei-2-105513, is arranged so that a capacitor element 21 consisting of a chip piece 21a and an anode pole 21b projecting away from the chip piece 21a are disposed between a pair of lead terminals 22 and 23 made from metal plates. The anode pole 21b of the capacitor element 21 is attached to an anode lead terminal 22 and after a cathode electrode film 21c has been formed on the outer surface of the chip piece 21a, a cathode lead terminal 23 is connected to the electrode film 21c through a safety fuse wire 24, such as a solder wire or the like. Thereafter, the entire structure is encapsulated in a molded casing 25 consisting of synthetic resin, and the lead terminals 22 and 23 are bent to engage the lower surface side of the resin casing 5.
In this conventional arrangement of a solid electrolytic capacitor with a safety fuse, the total length L thereof includes the length of the capacitor element 21, the length required to connect the lead terminal 22 to the anode pole 21b, and the length required to connect the chip piece 21a of the capacitor element 21 to the lead terminal 23 through the safety fuse wire 24. Similarly, the total height H thereof includes the height of the capacitor element 21 and the height of the safety fuse wire 24 projecting above the upper surface of the capacitor element 21.
Thus, in such a conventional solid electrolytic capacitor arrangement having a safety fuse, the ratio of the length of the capacitor element 21 to the total length L and, the ratio of the height of the capacitor element 21 to the total height H are so small that the ratio of the volume of the capacitor element to the total volume, that is, the volumetric efficiency of the capacitor element, is low. Accordingly, the solid electrolytic capacitor arrangement must be large. In addition, because two lead terminals made from metal plates are included, the weight is excessive.
Furthermore, in manufacturing a conventional solid electrolytic capacitor with a safety fuse as described above, the rate of occurrence of defective products is high, and the manufacturing cost is also quite high because a complex technique is required to connect a safety fuse wire such as a solder wire to a capacitor element and lead terminals.